Apparatus for scribing thin films in photovoltaic cells

ABSTRACT

A thin-film scribing apparatus employing an optical device converts a low M 2 , Gaussian or pseudo-Gaussian beam into an inverted Gaussian beam. The all refractive optical device is such that it is not susceptible to either beam size or angular variations and exhibits very little loss of energy for the transformation process. The output can be configured for either single or dual-axis operation where the geometric shape of the beam is rectangular or square with steep edge intensity. The resulting rectangular beam requires less beam overlap and has very little shoulder in the intensity profile, providing high uniformity scribe features with greatly improved processing speeds.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates, generally, to optics. More particularly, itrelates to scribing thin films in photovoltaic cells.

2. Description of the Prior Art

Conventional scribing of thin films used in solar cells uses focusedGaussian or Gaussian-like laser beams. A round Gaussian laser beam ispassed through a focusing lens. Typically, a singlet, best form lens, ora doublet is used, depending upon how tightly the user wants to focus.The price of the optic increases with increasing complexity of thefocusing objective.

The substrate to be scribed is placed at the focus of the lens. Thinfilms ablate from the substrate after the intensity of the focused laserbeam reaches a particular threshold. The laser energy absorbed prior toablation threshold goes into the material as heat, causing thermaldamage to the thin film and surrounding area.

Moreover, laser beams are predominately round. Accordingly, a largeoverlap of consecutive spots is required to minimize scalloping at theedge of the scribe line. This large overlaps slows down the scribingprocess considerably.

The homogenization of a laser beam can correct the non-steep sideprofile of a Gaussian distribution. However, homogenization is difficultto implement for scribing applications because the optics used in suchscribing systems tend to be flying optics; this precludes usingimaging-based homogenized laser beams.

A free form phase shifting optical device can produce a top hat profilethat would otherwise be desirable. However, such devices cannot toleratevariation to the input laser beam or beam misalignment. Accordingly,they have not found much use in production systems.

Focused laser beams that are predominately Gaussian or Gaussian-like inshape are not ideal where a steep energy transition is needed to cleanlyremove a thin film.

A free form phase shifting optical device such as disclosed in U.S. Pat.No. 6,295,168 transforms a Gaussian laser beam into a top-hathomogenized field but is difficult to implement in production due to thestringent requirements of an extremely stable laser beam and itsalignment into the optical system.

A laser beam homogenizer such as disclosed in U.S. Pat. No. 6,621,639inverts a small section of a Gaussian-like beam for better homogeneityafter it passes through a lens array homogenizer. This approach is animaged based solution limited to lasers with low spatial coherence anddoes not lend itself to systems that require a flying optics head.

U.S. Pat. No. 6,975,458 discloses an off-axis grating that transforms aGaussian distribution to create a specific shape in the far field. Thisapproach requires expensive optics and does not have high diffractionefficiency. Moreover, it is usually implemented as an imaging basedsystem which is not suitable for flying head optics.

U.S. Pat. No. 6,697,181 discloses an apparatus that creates an invertedGaussian shape at a scan focus using special transmission plates and apyramid type prism. It is not a collimated solution and is therefore notpractical for a scribing system.

U.S. Pat. No. 5,798,877 discloses rectangular prisms that break up andreposition a laser beam into two components in an effort to improve thesymmetry of a diode array. Several complex prisms are used to accomplishthe task.

An inexpensive but effective method for scribing a square spot insteadof a conventional round spot is needed. This would eliminate the needfor a large overlap of consecutive round spots and would eliminate thescalloping at the edge of the scribe line. It would also speed up thescribing process considerably.

However, in view of the prior art taken as a whole at the time thepresent invention was made, it was not obvious to those of ordinaryskill how the identified need could be fulfilled.

SUMMARY OF THE INVENTION

The long-standing but heretofore unfulfilled need for an apparatus thatimproves the scribing process by reducing the need for large overlappingof consecutive spots as required by the prior art to minimize scallopingat the edge of the scribe line is now met by a new, useful, andnon-obvious invention.

The novel apparatus is an all refractive optical device that transformsa Gaussian laser beam, a pseudo-Gaussian laser beam, or a low M² laserbeam into an inverted Gaussian laser beam. The novel optical device issubstantially not susceptible to beam size or angular variations. Itexhibits insubstantial loss of energy during the transformation of theGaussian laser beam, pseudo-Gaussian laser beam, or low M² laser beaminto the inverted Gaussian laser beam.

The novel optical device has an output configured for single-axisoperation where the geometric shape of the laser beam is rectangularwith steep edge intensity. It also has an output configured forsingle-axis operation where the geometric shape of the beam is squarewith steep edge intensity.

The novel optical device has an output configured for dual-axisoperation where the geometric shape of the beam is rectangular withsteep edge intensity. It also has an output configured for dual-axisoperation where the geometric shape of the beam is square with steepedge intensity.

A resulting rectangular beam requires less beam overlap and has verylittle shoulder in the intensity profile. High uniformity scribefeatures are produced and processing speeds are improved.

A first embodiment of the novel thin-film scribing apparatus includes atelescope including an array of four (4) spherical lenses. Each of thefour (4) spherical lenses are cut as square elements and placed inabutting relation to form a square array. The telescope has a 1×magnification so that an outer dimension of the beam is not altered.

In a second embodiment, a first glass cube is rotated forty five degrees(45°) on an X axis where an optical axis of laser light propagates alonga Z axis so that said first cube of glass inverts a laser beam in oneaxis and so that the laser beam inverted in one axis is sufficient forscribing in one axis of a solar panel.

In a variation of the second embodiment, a first glass cube is rotatedforty five degrees (45°) on a Y axis where an optical axis of laserlight propagates along a Z axis.

In a third embodiment, a second glass cube is in alignment with thefirst glass cube. The second glass tube is rotated forty five degrees(45°) from the opposing axis of the first glass tube to invert the laserbeam in both axes.

In a fourth embodiment, an octahedron optical element is adapted toinvert both axes of a Gaussian beam. The octahedron optical elementincludes a polygon refractor having a plurality of flat surfaces. Eachflat surface of the polygon refractor is coated with an anti-reflectioncoating to minimize surface reflection losses for a laser wavelengthused for scribing.

A fifth embodiment includes a first pair of dove prisms having silveredbases disposed in base-to-base abutting relation to one another. In avariation of the fifth embodiment, a second pair of dove prisms isdisposed in base-to-base abutting relation to the first pair of doveprisms. The first pair of dove prisms is rotated ninety degrees (90°)with respect to the second pair of dove prisms. The first and secondpairs of dove prisms are equivalent to two cubes placed in base-to-baseabutting relation to one another where a first cube is rotated ninetydegrees (90°) with respect to a second cube.

All embodiments include a thin film substrate, a laser adapted toprovide a laser beam, a refractive optics with suitable antireflectioncoating to minimize reflection losses positioned in a path of the laserbeam so that a center of the laser beam passes through a center of therefractive optics, and a focusing means positioned between saidrefractive optics and the thin film substrate to focus the laser beamonto the thin film.

The refractive optics may take the form of a 4×4 element Kepleriantelescope, a single glass cube, dual cubes, an octahedron, or dual doveprism combinations.

The inventive structure produces a square profile with a very sharpintensity slope at the edge of the profile. The square profile iscollimated so that it can be focused by a focusing object that can beplaced anywhere beyond the prism assembly.

An important object of the invention is to increase the intensity of alaser beam on the outside of the focused laser spot regardless of wherethe focus optics are positioned in the system.

Another important object is to create a rectangular focused spot areawithout special imaging.

Still another object is to produce a laser scribe that is free ofscalloping or lifting at the edges.

These and other important objects, advantages, and features of theinvention will become clear as this disclosure proceeds.

The invention accordingly comprises the features of construction,combination of elements, and arrangement of parts that will beexemplified in the description set forth hereinafter and the scope ofthe invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and objects of the invention,reference should be made to the following detailed description, taken inconnection with the accompanying drawings, in which:

FIG. 1A is a graph depicting the intensity of a Gaussian laser beam inwatts per square centimeters as a function of distance in millimetersfrom the center of the beam;

FIG. 1B is a grey scale, cross-sectional view, intensity profile of theGaussian laser beam of FIG. 1A;

FIG. 1C is a graph having the same parameters as the graph of FIG. 1Abut depicting the intensity of a Gaussian laser beam that has beentransformed by the all-refractive optics of this invention;

FIG. 1D is a grey scale, cross-sectional view, intensity profile of theGaussian laser beam of FIG. 1C;

FIG. 2A is a side elevational view of a first embodiment of the noveloptics;

FIG. 2B is a top plan view of the first embodiment;

FIG. 2C is an end view of the first embodiment;

FIG. 3A is a side elevational view of a glass cube embodiment;

FIG. 3B is a top plan view of the structure depicted in FIG. 3A;

FIG. 3C is a grey scale, cross-sectional view, intensity profile of aGaussian beam that has passed through the glass cube of FIGS. 3A and 3B;

FIG. 4A is a side elevational view of a third embodiment;

FIG. 4B is a top plan view of the third embodiment;

FIG. 4C is a grey scale, cross-sectional view, intensity profile of aGaussian beam that has passed through the apparatus of FIGS. 4A and 4B;

FIG. 5A is a perspective view of a octahedron optical element formed bytwo pyramidal optical elements disposed in base-to-base abuttingrelation to one another;

FIG. 5B is a side elevational view of said octahedron optical element;

FIG. 6 is a graph depicting the intensity of a Gaussian laser beam inwatts per square centimeters as a function of distance in millimetersfrom the center of the beam that has been transformed by said octahedronoptical element at the focus of a positive lens;

FIG. 7 is a side elevational view of an optical element formed by twodove prisms disposed in base-to-base relation to one another;

FIG. 8 depicts an octahedron Gaussian inverter system for thin filmscribing;

FIG. 9A is a perspective view of a dual dove prism arrangement for atwo-axis Gaussian inversion;

FIG. 9B is a side elevational view of the structure depicted in FIG. 9A;

FIG. 10A is a prior art view of a round spot scribe having scallopededges; and

FIG. 10B depicts the efficient scribe of square shapes made possible bythe inventive structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1A graphically plots an arbitrary intensity in watts per squarecentimeter of a Gaussian or Gaussian-like laser beam on a Y-axis againstthe distance in millimeters from the center of the beam on an X-axis.

FIG. 1B depicts such a beam in a cross-sectional view. As indicated inthe graph and the drawing, the intensity of the beam is highest at itscenter and decreases as radial distance from said center increases untilthe lowest density is at the peripheral edge of the beam.

FIG. 1C graphically plots the intensity in watts per square centimeterof a Gaussian or Gaussian-like laser beam on a Y-axis against thedistance in millimeters from the center of the beam on an X-axis aftersaid beam is transformed by a Gaussian inverter.

FIG. 1D depicts the beam in a cross-sectional view after suchtransformation.

A Gaussian inverter is a refractive optic that transforms a Gaussian orGaussian-like laser beam into an inverted Gaussian shape so that thehigh intensity at the peak of the Gaussian beam is moved to the outsideand the low intensity side lobes are moved into the center of the beam.

This is accomplished in a first embodiment of the invention by making aspecial telescope 10 that includes an array of four (4) spherical lensesas depicted in FIGS. 2A, 2B, and 2C. The spherical lenses, denoted 12,14, 16, 18, are cut as square elements and placed side by side. Thetelescope is a Keplerian type with 1× magnification so as not to alterthe outer dimension of the beam.

FIG. 2A depicts the telescope in side elevation, FIG. 2B provides a topplan view, and FIG. 2C is an end view thereof. Although the 4×4Keplerian telescope would in theory produce the desired invertedGaussian beam, it is difficult to manufacture the lenses with enoughaccuracy to avoid aberrations and asymmetry in the inverted Gaussianprofile. The lenses add power to the beam and alignment for collimationis paramount for good functionality.

FIGS. 3A and 3B depict a second embodiment in side and top planelevation, respectively, where a cube of glass 20 is rotated forty fivedegrees (45°) on either an X axis or Y axis, assuming the optical axisof the laser light propagates along the Z axis. A single cube of glassinverts the laser beam in one axis and this alone is sufficient forscribing in one axis of a solar panel with good results. FIG. 3C depictsthe beam pattern, as a cross-sectional view grey scale intensityprofile, produced by this apparatus.

FIGS. 4A and 4B depict a third embodiment in side and top planelevation, respectively, where a second cube of glass 22 is added andequally rotated forty five degrees (45°) about the X axis of the firstrotated prism 10 and then further rotated ninety (90°) about the Z(optical) axis to invert in both axes. The beam pattern produced by thisapparatus is depicted in FIG. 4C.

The preferred embodiment is a single piece of glass such as anoctahedron optical element 24 depicted in FIG. 5A that inverts both axesof the Gaussian beam as depicted in FIG. 5B. Each surface of the polygonrefractor is coated to minimize surface reflection losses with asuitable anti-reflection coating for the laser wavelength being used forscribing. Each octahedron has eight inclined, triangular sides.

The grey scale intensity profile of the apparatus depicted in FIGS. 5Aand 5B is depicted in FIG. 1D.

FIG. 6 graphically depicts a cross sectional intensity profile of afocused spot of light passed through a octahedron Gaussian Inverter andfocused with a f=100 mm plano-convex lens. This focused spot isnominally 30 microns wide and is typical for a thin film scribe. Theintensity of the spot is high and steep at the edge and much lower thanthe center which corresponds to the basic shape of the inverted Gaussianbeam. In scribing thin films the steep and high intensity edge of thespot easily and cleanly ablates. The center of the spot, althoughgenerally below ablation threshold, still gets removed through liftingof the material from the adjacent high intensity edges of the beam. Inaddition, there are shock wave forces from the ablation, thereby furtheradding the lifting of the central region. Since the refractive prismapproach has no optical power, the inverted beam is very focusable anddoes not contribute wave front aberrations that would be transferred tothe focusing lens. It is, however, important that the facets of the cubeor octahedron glass have good orthogonality and parallelism. It is notdifficult to achieve parallelism and orthogonality of better than ten(10) arc seconds. A ten (10) arc second wedge in a glass plate having anindex of refraction of 1.5 would cause a three (3) micron displacementof the focal spot with a one hundred millimeter (100 mm) focal lengthlens. A typical focal spot of a laser beam for scribing is greater thanthirty (30) microns and therefore such a shift would have no adverseconsequence to the laser scribe.

A pair of dove prisms 26 a, 26 b, depicted in FIG. 7, with silveredbases placed base-to-base could achieve the same result as a singlecube. There is no real advantage to manufacture such an arrangementbecause a single cube is much more efficient to fabricate.

Two sets of dove prisms arranged base-to-base and rotated ninety degrees(90°) from one pair to the other would be the equivalent of two cubesplaced in a similar fashion as depicted in FIGS. 9A and 9B.

The grey scale intensity profile created by the apparatus of FIG. 7 isdepicted in FIG. 3C. Accordingly, glass cube 20 and dove prisms 26 a, 26b, produce substantially the same grey scale intensity profile.

FIG. 8 depicts an octahedron Gaussian inverter system, denoted 28 as awhole. It includes a laser having a Gaussian or Gaussian-like transverseshape that is directed onto refractive means 30 such as an octahedron asdepicted in FIG. 8. However, a 4×4 element Keplerian telescope, dualcubes and dual dove prism combinations achieve the same result butrequire more elements. Focusing element 32 focuses the light onto thinfilm substrate 34.

FIGS. 9A and 9B provide perspective and side elevational views,respectively, of a dual dove prism arrangement for a 2 axis Gaussianinversion, denoted 36 as a whole. Item 38 is a dual dove prism and item40 is a focusing element.

FIG. 10A depicts round spot scribes having scalloped edges and FIG. 10Bdepicts the more efficient square spot scribes made possible by thepresent invention.

The Gaussian inverter optic is not limited to thin film scribing. Itcould easily be used in applications where square, rectangular, orsimilar shapes need to be drilled in a material and could also aidstraight line cutting applications. Adding a rotational means to theGaussian inverter enables its steep edge profile to be used for scribingor cutting at angles or curves. The rotation of the Gaussian invertercan be synchronized with the linear travel of the stage or scanning ofthe laser beam to accommodate arcs and curves. Moreover, some weldingapplications could benefit from the inverted Gaussian profile tominimize key holing effects.

It will thus be seen that the objects set forth above, and those madeapparent from the foregoing description, are efficiently attained andsince certain changes may be made in the above construction withoutdeparting from the scope of the invention, it is intended that allmatters contained in the foregoing description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention that, as amatter of language, might be said to fall therebetween.

What is claimed is:
 1. A thin-film scribing apparatus, comprising: afirst glass cube having an initial, unrotated position, a length of saidfirst glass cube extending along a horizontal Z axis, a height of saidfirst glass cube extending along a vertical Y axis in a common planewith said Z axis, and a width of said first glass cube extending alongan X axis that is oriented at a ninety degree (90°) angle relative tosaid common plane of said Y and Z axes, said first glass cube beingrotated forty five degrees (45°) relative to said X axis in said commonplane so that it is intermediate said Y and Z axes where an optical axisof a laser beam propagates along said Z axis; whereby all of the laserbeam passes through the first glass cube without loss of power; wherebysaid first glass cube inverts said laser beam in one axis; whereby saidlaser beam inverted in said one axis is sufficient for scribing in oneaxis of a solar panel; and whereby the optical path of the thin-filmscribing apparatus is minimized.
 2. A thin-film scribing apparatus,comprising: a first glass cube having an initial, unrotated position, alength of said first glass cube extending along a horizontal Z axis, aheight of said first glass cube extending along a vertical Y axis thatis oriented at a ninety degree (90°) angle from said horizontal Z axisin a common plane with said Y axis, and a width of said first glass cubeextending along an X axis that is oriented at a ninety degree (90°)angle relative to said common plane of said Y and Z axes, said firstglass cube being rotated forty five degrees (45°) relative to said Yaxis in said common plane so that it is intermediate said X and Z axeswhere an optical axis of a laser beam propagates along said Z axis;whereby all of the laser beam passes through the first glass cubewithout loss of power; whereby said first glass cube inverts said laserbeam in one axis; whereby said laser beam inverted in one axis issufficient for scribing in said one axis of a solar panel; and wherebythe optical path of the thin-film scribing apparatus is minimized. 3.The apparatus of claim 1, further comprising: a second glass cube inalignment with said first glass cube; said second glass cube rotatedforty five degrees (45°) from the opposing axis of the first glass cubeto invert the laser beam in both axes.
 4. The apparatus of claim 2,further comprising: a second glass cube in alignment with said firstglass cube; said second glass cube rotated forty five degrees (45°) fromthe opposing axis of the first glass cube to invert the laser beam inboth axes.